1. Field of the Invention
This invention relates to an image forming apparatus used in recording processes utilizing electrophotography, electrostatic recording and so forth. More particularly, it relates to an image forming apparatus which has a powder transport mechanism intended to replenish a toner (or a mixture of a toner and a carrier) to a developing assembly, and transports the powder especially in a high precision to enable stable and high-quality image formation.
2. Related Background Art
In recent years, machinery making use of electrophotography is applied in apparatus such as facsimile machine and printers in addition to conventional copying machines. As coming to be applied in a broader range, the machinery has come to be strongly desired to achieve high-speed, long-lifetime, high-stability and full-color image formation while having compactness, low cost and high image quality.
In particular, as a need for full-color electrophotography, the number of reproduction has increased because of reproduction of maps, design pictures, photographs and so forth, and it is called for them to have good color reproduction and be very minutely and faithfully reproduced without crushed images or broken-off images over details.
In most developing assemblies of full-color image forming apparatus, two-component developers are used from the viewpoint of color development and color mixing performance. As well known, in order to achieve stable developing performance, it is an important factor that the mixing ratio of toner to carrier (hereinafter “T/C ratio”) of a two-component developer is kept at a certain value. The toner of a developer is consumed at the time of development, and the T/C ratio of the developer changes. Hence, it is necessary to opportunely detect the T/C ratio of the developer in a developing assembly by means of a toner concentration control unit (ATR) so that the toner can be fed in accordance with its changes to always constantly control the mixing ratio in the developer to keep the quality level of images.
Now, it is very often seen to be so structured that the toner to be fed to the developing assembly is replenished from a toner cartridge through a transport section which is a powder transport mechanism. Also, in the powder transport mechanism, it has been common to use such a structure that the transport quantity is controlled using a transport screw which is a spiral auger. This is largely because of such advantages that the assembly can be made simple in structure and the transport quantity in one rotation of screw can be defined with ease to enable achievement of necessary controlled quantity in inexpensive structure (disclosed in, e.g., Japanese Patent Application Laid-open No. 2001-134053, No. H06-161246 and No. H06-083196).
However, at the present time that it is desired to reproduce at a high speed and stably images having various image percentages, ranging from images having a large image area as in photographs up to images having a small image area as in one-point colors, the toner has come active in its movement at the part of a clearance present between a toner transport pipe which forms an outer wall of the toner transport screw and the outer diameter of the toner transport screw. This has caused a flashing phenomenon that the toner slips through the clearance like water and therefore the toner is unwantedly fed in excess to the developing assembly.
In particular, in a full-color image forming apparatus having what is called a rotary unit, which is preferably used as an inexpensive and compact full-color image forming apparatus, a flashing phenomenon accompanying the rotation and stop impact of the rotary unit has occurred in some cases.
As a means for preventing such a flashing phenomenon, as disclosed in Japanese Patent Application Laid-open No. H05-224530, a method is proposed in which the clearance between the toner transport screw and the toner transport pipe is controlled.
However, at the present time that a desire for low-temperature fixing of toner is put forward from the viewpoint of energy saving, masses of toner may be formed to cause defective images, because, if the clearance is in a certain value or less, the clearance between the toner transport screw and the toner transport pipe comes to be lost because of mechanical characteristics such as eccentricity or run-out of the transport screw, so that the toner is rubbed between the toner transport screw and the toner transport pipe.
Even if the flashing phenomenon can be somehow prevented by spending production costs, e.g., by controlling the run-out or making higher the precision of inner diameter of the toner transport pipe, it has been still insufficient for achieving replenishment stability and precision which are high enough to continue to maintain a good stability for high image quality.
In particular, with an increase in toner consumption per unit time that is brought by making process speed higher, the toner transport screw is being made to be rotated at a higher speed. Under such circumstances, any image forming apparatus has not yet been found which has a toner transport mechanism having a simple structure, exhibiting always stable toner transport performance and promising superior stability for high image quality.